'At Right Angles' printed from http://nrich.maths.org/

It's easy to draw parallel lines - just check that the gradients
match.

Gradient $\frac{3}{2}$

Gradient $0$

Gradient $-2$

But I'm finding it harder to draw perpendicular lines. Here are my
best efforts so far but I don't think they're quite right!

I know that the sides of a square are at right angles, so if I
learn to draw tilted squares I may be able to find an efficient
method for drawing perpendicular lines.

Experiment with the interactivity below until you can draw squares
with confidence.
Work out the gradients of the lines which form your squares.
Is there a relationship between the gradients of perpendicular
lines?
Can you use your relationship to explain why the two sets of lines
above are not perpendicular?